A rule of Shva which modern Hebrew speakers [unfortunately] have forgotten is the following:
The Shva in this case borrows the sound of the Qamats from the very next letter.
etc..
However, if it is joined with one of four [guttural] letters, א ח ה ע, its manner [of pronunciation] will be like the manner of the vowel of the second letter in that word
וַיֹּ֣אמֶר אֱלֹהִ֗ים יְהִ֤י מְאֹרֹת֙ בִּרְקִ֣יעַ הַשָּׁמַ֔יִם לְהַבְדִּ֕יל בֵּ֥ין הַיֹּ֖ום וּבֵ֣ין הַלָּ֑יְלָה וְהָי֤וּ לְאֹתֹת֙ וּלְמֹ֣ועֲדִ֔ים וּלְיָמִ֖ים וְשָׁנִֽים
etc..
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